The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
(1) Field of the Invention
This invention generally relates to a technique for turbulent boundary layer thickness estimating using hot film wall shear stress sensors.
More particularly, the invention relates to a technique for estimating turbulent boundary layer thickness in an underwater environment using hot film shear wall stress sensors and correlation coefficients.
(2) Description of the Prior Art
The art for hot wire anemometry has been widely used since the 1950""s as a technique for making measurements of velocity and shear stress in experimental fluid mechanics facilities. Non-intrusive hot film sensors were developed in the late 1960""s to measure the wall shear or tangential stress. These sensors take advantage of the relationship between the rate of heat transfer from small thermal elements and the local wall shear stress. The wall shear stress is related to the velocity gradient at the wall by the relation:                                           τ            =                          μ              ⁢                              xe2x80x83                            ⁢                                                ∂                  u                                                  ∂                  y                                                              "RightBracketingBar"                          y          =          0                                    (        1        )            
Since the metal film used is adhered to a hard backing or substrate, the sensor is remarkably robust and useful for underwater applications. Whereas pressure sensors are typically used in both laboratory and real-world settings, hot film sensors have not been implemented as a diagnostic measurement tool on actual underwater or surface vehicles. This invention proposes to extend the range of applications to cases including at-sea testing and tactical operations. Currently, no non-intrusive measurement techniques exist for quantifying the turbulent boundary layer thickness outside of a laboratory environment.
The following patents, for example, disclose devices for detecting turbulent flow, but do not disclose the use of hot film sensors and correlation functions for measuring a turbulent boundary layer as disclosed in the present invention.
U.S. Pat. No. 4,188,823 to Hood;
U.S. Pat. No. 4,350,757 to Montag et al.;
U.S. Pat. No. 4,774,835 to Holmes et al.;
U.S. Pat. No. 4,993,261 to Lambert;
U.S. Pat. No. 5,272,915 to Gelbach et al.; and
U.S. Pat. No. 5,890,681 to Meng.
Specifically, Hood discloses a system for detecting the laminar to turbulent boundary layer transition on a surface while simultaneously taking pressure measurements. The system uses an accelerometer for producing electrical signals proportional to the noise levels along the surface and a transducer for producing electrical signals proportional to pressure along the surface. The signals generated by the accelerometer and transducer are sent to a data reduction system for interpretation and storage.
The patent to Montag et al. discloses a method for making visible by photochemical means residual moisture distributions in, photographic wet film layers subjected to a gas flow. According to the invention, a film diffusely pre-exposed is immersed in an aqueous swelling agent solution which contains either (a) a reducing agent or (b) an alkali. After being exposed to the air stream, the invisible residual moisture profile is immersed in an alcoholic solution of either (a) an alkali or (b) a reducing agent. The half-tone image produced serves for determining stationary local boundary layer thickness distributions, wall shearing stresses, material transfer coefficients and heat transfer coefficients.
Holmes et al. discloses a method of visualizing laminar to turbulent boundary layer transition, shock location, and laminar separation bubbles around a test surface. A liquid crystal coating is formulated using an unencapsulated liquid crystal operable in a temperature bandwidth compatible with the temperature environment around the test surface. The liquid crystal coating is applied to the test surface, which is preferably pre-treated by painting with a flat black paint to achieve a deep matted coating, after which the surface is subjected to a liquid or gas flow. Color change in the liquid crystal coating is produced in response to differences in relative shear stress within the boundary layer around the test surface.
Lambert discloses a fluid flow meter including a sensor mounted on or in the inner surface of a conduit for measuring fluid flow through the conduit where the sensitivity of the sensor is dependent upon the thickness of the fluid boundary layer extending over the sensor. According to the invention, fluid is drawn out of the conduit through an aperture located a predetermined distance upstream of the sensor to remove the boundary layer developed upstream of the sensor thereby rendering the sensor immune to fluctuations in the thickness of the removed boundary layer. At the same time, a fresh boundary layer of reduced thickness and greater stability is initiated over the sensor so as to improve the sensitivity and repeatability of the sensor.
The patent to Gelbach et al. discloses an airflow sensing system for determining the type of airflow flowing over a flight surface. A hot film sensor is driven by a constant voltage feedback circuit that maintains the voltage across the sensor at a predetermined level. A signal processing circuit receives an output signal of the feedback circuit and determines whether the output signal is indicative of laminar, transitional, or turbulent airflow. The transitional airflow is distinguished form turbulent airflow by a signal having significant energy in a low-frequency pass band from 50-80 Hz. The signal processing circuit drives a three-color LED display to provide a visual indication of the type of airflow being sensed.
Meng discloses a method for controlling microturbulence in a medium flowing near a surface. The method includes the steps of measuring the forces acting near or on the surface and using those measurements to determine the state probabilities for the microturbulent events occurring at the surface. The control method then activates selective cells in an array of cells to apply forces at the surface to counteract the microturbulent events and thus reduce turbulence. Each cell has a pair of electrodes and opposing magnetic poles such that when the control method activates a cell, the interaction of the electric field and the magnetic field at the cell creates a Lorentz force normal to the surface.
It should be understood that the present invention would in fact enhance the functionality of the above patents by providing a unique concept for estimating the thickness of a hydrodynamic turbulent boundary layer on undersea vehicles, surface vessels, towed bodies or in a laboratory setting. It utilizes commercially available hot film sensors to non-intrusively measure the thickness of the turbulent boundary layer on a surface.
Therefore it is an object of this invention to provide a method for measuring a turbulent boundary layer thickness.
Another object of this invention is to provide a method for measuring a turbulent boundary layer thickness using hot film wall shear stress sensors.
Still another object of this invention is to provide a method for measuring a turbulent boundary layer thickness utilizing sensor measurements and correlation coefficients.
A still further object of the invention is to provide a method for measuring turbulent boundary layer thickness in underwater applications.
In accordance with one aspect of this invention, there is provided a method and apparatus for determining turbulent boundary layer thickness. Specifically, a pair of sensors are mounted to a solid surface interfacing with a fluid at two separate stream wise locations. A voltage output from the pair of sensors is recorded and a real non-dimensional value of a correlation coefficient is computed with measured data from the recorded voltage. A laboratory non-dimensional value of the correlation coefficient is independently determined from laboratory data. The real non-dimensional value is compared with the laboratory non-dimensional value to obtain a boundary layer thickness having a value which minimizes a difference between the values of the real non-dimensional value and the laboratory non- dimensional value.